Chemicals that are used to accelerate the vulcanization process may be referred to as accelerator chemicals, rubber accelerators or plastic accelerators. Such chemicals accelerate the vulcanization or curing process during which cross-linking reactions take place that cause the product rubbers, plastics or rubber/plastic blends to have a desired set of characteristics. The accelerator chemicals, rubber accelerators or plastic accelerators may also, therefore, be referred to as cross-linking agents.
Chemicals that are typically used to accelerate the rubber vulcanization process are toxic. Many such chemicals cause defects by bleeding out of the rubber compound after having been cured. Many accelerator chemicals cause staining and discoloring of light-colored rubber compounds. Certain widely used accelerators lack storage stability, breaking down chemically after 4-6 months. Many accelerator chemicals have foul, pungent odors and are difficult to work with.
For many rubbers such as EPDM rubbers (which are ethylene-propylene-diene-monomer-based rubber compounds), nitrile rubber, Hypalon and Neoprene, ultra and secondary accelerators are required in addition to the primary accelerator to quickly obtain full cure. Two to as many as four of the ultra and one of the secondary accelerators are typically used in small amounts, usually from about 1 wt % to 2 wt %. These chemicals are toxic and very costly and, furthermore, pose a difficult problem to admix correctly and uniformly into rubber compounds in the small amounts required. If a slightly higher amount, on the order of 0.25 wt %, of an ultra accelerator is inadvertently added into the rubber compound, the rubber compound scorches badly during processing and becomes overcured, causing lower physical properties than what is typically required or desired. Usually such rubber batches are scraped as a total loss.
If the amount of the ultra accelerators is slightly lower than what is required the rubber compound becomes undercured. This causes the rubber compound to have physical properties of lower quality than what is required in properly cured rubber compounds. Such batches are also usually scraped or offered at much lower prices as off-specification.
For EPDM and nitrile rubbers, two to four ultra accelerators are typically required in addition to the primary accelerator so as to obtain fast, complete cure. The ultra accelerators are usually captax, methyl tuads, sulfads, ethyl tellurac, as made, for example, by R. T. Vanderbilt. The primary accelerator is usually Santocure NS, as provided by Monsanto.
To cure Hypalon (made by E. I. Dupont) a high percentage of litharge (lead monoxide, PbO) is typically required, that is, about 10%. Since litharge is a lead-based chemical, it is very toxic. Furthermore, litharge tends to cause rubber compounds to become very scorchy during processing. In addition, Hypalon typically requires additional accelerators, plus the litharge vulcanizing agent, to obtain a fast and complete cure. The secondary accelerators that are typically used are zinc oxide and Tetrone A.
For neoprene rubber, which tends to be scorchy during processing, zinc oxide and Magalite D are typically used to obtain a fast and complete cure. During such processing steps undesirable toxic fumes may be produced.